HWUI: calculate used memory in FontCache for gfxinfo

bug:30427106
Change-Id: I653571d6a4e974e975fb0dc03fc2364eecbf2f84
diff --git a/libs/hwui/font/CacheTexture.h b/libs/hwui/font/CacheTexture.h
index 4dfb41d..6750a8a 100644
--- a/libs/hwui/font/CacheTexture.h
+++ b/libs/hwui/font/CacheTexture.h
@@ -178,6 +178,8 @@
         return mCurrentQuad == mMaxQuadCount;
     }
 
+    uint32_t calculateFreeMemory() const;
+
 private:
     void setDirty(bool dirty);